Some available domains have been previously registered and are now expired. You can register
them again. The Traffix Rank indicates, that the domain is still receiving some DNS traffic according to Verisign.

How does it work? ?

The data is provided by Verisign, that operates the .com and .net registry.
Verisign computes scores on a 1-10 scale (10 being
best), to measure the reliability and quantity of DNS traffic for a given domain.

Second level Non-existent domain (NXD) traffic includes traffic to the top-level
Domain Name Servers where either the second-level domain name being queried does
not exist or the domain name does exist but does not properly have its DNS settings
configured. The NXD traffic includes traffic from humans (e.g. type in traffic,
clicking on a bookmark to a site that no longer exists, etc). But, every TLD's NXD
traffic also includes machine traffic such as bots and search crawlers.